In the rolling of strip and sheet materials it is common practice to roll a material to the desired dimensions in a cold or hot rolling mill stand and then feed the resulting strip to a coiler where it is wound up into a coil. In FIG. 1 a flatness control is illustrated that typically comprises of three main parts, measurement, control and actuators, that correct the flatness error. The difficulty with the flatness control consists on achieving a very good flatness in spite of certain known limitations. The reason why the flatness is not always perfect, is that all three parts have a number of limitations.
The measurement is for example often discrete both across the width of the strip, as it has a limited number of measurement zones, and along the strip, as there are only a limited number of measurements done per rotation (say, four measuring zones in the rotational direction). The control has a complex mathematics that cannot always follow at the speed of the strip and the actuators are only correcting certain type of errors and it is not sure they can correct any shape of flatness error. For example, a bending and a tilting actuator can correct only a shape that is a sum of a parable and a straight line, but of course the flatness error can in principle have any irregular shape.
A metal strip that is subject to different degrees of reduction across its width will be elongated in varying length over different sections of the strip. Normally, the reduction results in certain pre-defined stress profiles for the strip.
The difference between the measured strip flatness and a target flatness curve is defined as the “flatness error”. Practically, the flatness error of the strip can be gauged by a special measuring roll, e.g. a Stressometer roll, an ABB product.